fix: compilation errors with musl

[jpnurmi]

This PR fixes the following compile errors with musl:

1. crashpad includes glibc-specific <sys/cdefs.h> without appropriate __GLIBC__ guard

   In file included from sentry-native/external/crashpad/util/linux/scoped_ptrace_attach.cc:17:
   sentry-native/external/crashpad/compat/linux/sys/ptrace.h:20:10: fatal error: 
   sys/cdefs.h: No such file or directory
      20 | #include <sys/cdefs.h>
         |          ^~~~~~~~~~~~~

2. crashpad uses pread64, off64_t etc. that are only available in musl when _LARGEFILE64_SOURCE is defined

   In file included from sentry-native/external/crashpad/util/process/process_memory_linux.cc:27:
   sentry-native/external/crashpad/util/process/process_memory_linux.cc: In lambda function:
   sentry-native/external/crashpad/util/process/process_memory_linux.cc:48:24: error: 'pread64' was not declared in this scope; did you mean 'pread'?
      48 |           HANDLE_EINTR(pread64(mem_fd_.get(), buffer, size, address));
         |                        ^~~~~~~
   
   In file included from sentry-native/external/crashpad/util/linux/memory_map.cc:15:
   sentry-native/external/crashpad/util/linux/memory_map.h:46:5: error: 'off64_t' does not name a type; did you mean 'off_t'?
      46 |     off64_t offset;
         |     ^~~~~~~
         |     off_t

These fixes are needed for getsentry/sentry-native#1233 to pass the CI on Alpine Linux with musl.

Ref: getsentry/sentry-native#1232