Add AT_* constants for musl libc

[ericonr]

target: x86_64-unknown-linux-musl

minidump_linux_writer (part of firefox) fails to build on musl because these constants are missing (the getregs issue is separate):

 5:39.87    Compiling minidump_writer_linux v0.1.0 (https://github.com/msirringhaus/minidump_writer_linux.git?rev=9191af36343846b2c7ada65b9602b481b717c4d8#9191af36)
 5:40.22 error[E0425]: cannot find value `AT_NULL` in crate `libc`
 5:40.22     --> /builddir/firefox-87.0/third_party/rust/minidump_writer_linux/src/auxv_reader.rs:98:29
 5:40.22      |
 5:40.22 98   |             at_null = libc::AT_NULL;
 5:40.22      |                             ^^^^^^^ help: a constant with a similar name exists: `PT_NULL`
 5:40.22      |
 5:40.22     ::: /builddir/firefox-87.0/third_party/rust/libc/src/unix/linux_like/linux/mod.rs:1614:1
 5:40.22      |
 5:40.22 1614 | pub const PT_NULL: u32 = 0;
 5:40.22      | --------------------------- similarly named constant `PT_NULL` defined here
 5:40.22 error[E0425]: cannot find value `AT_PHDR` in crate `libc`
 5:40.22     --> /builddir/firefox-87.0/third_party/rust/minidump_writer_linux/src/dso_debug.rs:91:25
 5:40.22      |
 5:40.22 91   |         at_phdr = libc::AT_PHDR;
 5:40.22      |                         ^^^^^^^ help: a constant with a similar name exists: `PT_PHDR`
 5:40.22      |
 5:40.22     ::: /builddir/firefox-87.0/third_party/rust/libc/src/unix/linux_like/linux/mod.rs:1620:1
 5:40.22      |
 5:40.22 1620 | pub const PT_PHDR: u32 = 6;
 5:40.22      | --------------------------- similarly named constant `PT_PHDR` defined here
 5:40.22 error[E0425]: cannot find value `AT_PHNUM` in crate `libc`
 5:40.22   --> /builddir/firefox-87.0/third_party/rust/minidump_writer_linux/src/dso_debug.rs:92:26
 5:40.22    |
 5:40.22 92 |         at_phnum = libc::AT_PHNUM;
 5:40.22    |                          ^^^^^^^^ not found in `libc`
 5:40.22 error[E0425]: cannot find function `getregs` in module `ptrace`
 5:40.22    --> /builddir/firefox-87.0/third_party/rust/minidump_writer_linux/src/linux_ptrace_dumper.rs:121:32
 5:40.22     |
 5:40.22 121 |             let regs = ptrace::getregs(pid);
 5:40.22     |                                ^^^^^^^ not found in `ptrace`
 5:40.22 error[E0425]: cannot find value `AT_ENTRY` in crate `libc`
 5:40.22    --> /builddir/firefox-87.0/third_party/rust/minidump_writer_linux/src/linux_ptrace_dumper.rs:241:30
 5:40.22     |
 5:40.22 241 |             at_entry = libc::AT_ENTRY;
 5:40.22     |                              ^^^^^^^^ not found in `libc`

They are defined in include/elf.h on musl, and are defined for glibc in

libc/src/unix/linux_like/linux/gnu/mod.rs

Lines 1227 to 1251 in bd7e052

	pub const AT_NULL: ::c_ulong = 0;
	pub const AT_IGNORE: ::c_ulong = 1;
	pub const AT_EXECFD: ::c_ulong = 2;
	pub const AT_PHDR: ::c_ulong = 3;
	pub const AT_PHENT: ::c_ulong = 4;
	pub const AT_PHNUM: ::c_ulong = 5;
	pub const AT_PAGESZ: ::c_ulong = 6;
	pub const AT_BASE: ::c_ulong = 7;
	pub const AT_FLAGS: ::c_ulong = 8;
	pub const AT_ENTRY: ::c_ulong = 9;
	pub const AT_NOTELF: ::c_ulong = 10;
	pub const AT_UID: ::c_ulong = 11;
	pub const AT_EUID: ::c_ulong = 12;
	pub const AT_GID: ::c_ulong = 13;
	pub const AT_EGID: ::c_ulong = 14;
	pub const AT_PLATFORM: ::c_ulong = 15;
	pub const AT_HWCAP: ::c_ulong = 16;
	pub const AT_CLKTCK: ::c_ulong = 17;
	// AT_* values 18 through 22 are reserved
	pub const AT_SECURE: ::c_ulong = 23;
	pub const AT_BASE_PLATFORM: ::c_ulong = 24;
	pub const AT_RANDOM: ::c_ulong = 25;
	pub const AT_HWCAP2: ::c_ulong = 26;
	pub const AT_EXECFN: ::c_ulong = 31;

[ericonr]

Thanks!