include/err: Enforce c linkage for err and warn functions.

[g2gps]

Summary

This change ensures that xx_func names are not mangled by a c++ compiler and can be correctly resolved when loaded at runtime.

Impact

c++ applications which use err can be loaded correctly.

Testing

The issue without this addition can be reproduced by following:

1. Use the err function from a c++ application:

diff --git a/examples/helloxx/helloxx_main.cxx b/examples/helloxx/helloxx_main.cxx
index c64891de1..93a38be58 100644
--- a/examples/helloxx/helloxx_main.cxx
+++ b/examples/helloxx/helloxx_main.cxx
@@ -26,6 +26,7 @@
 
 #include <cstdio>
 #include <debug.h>
+#include <err.h>
 
 //***************************************************************************
 // Definitions
@@ -124,6 +125,8 @@ extern "C" int main(int argc, FAR char *argv[])
   g_HelloWorld.HelloWorld();
 #endif
 
+  err(11, "This produces an undefined symbol\n");
+
   delete pHelloWorld;
   return 0;
 }

2. Add BINFORMAT debugging and add helloxx application to rv-virt:knsh32:

diff --git a/boards/risc-v/qemu-rv/rv-virt/configs/knsh32/defconfig b/boards/risc-v/qemu-rv/rv-virt/configs/knsh32/defconfig
index 86d381bd07..f4bdd20da5 100644
--- a/boards/risc-v/qemu-rv/rv-virt/configs/knsh32/defconfig
+++ b/boards/risc-v/qemu-rv/rv-virt/configs/knsh32/defconfig
@@ -44,10 +44,18 @@ CONFIG_ARCH_USE_S_MODE=y
 CONFIG_BOARD_LOOPSPERMSEC=6366
 CONFIG_BUILD_KERNEL=y
 CONFIG_DEV_ZERO=y
+CONFIG_DEBUG_BINFMT=y
+CONFIG_DEBUG_BINFMT_ERROR=y
+CONFIG_DEBUG_ERROR=y
+CONFIG_DEBUG_FEATURES=y
+CONFIG_DEBUG_INFO=y
+CONFIG_DEBUG_WARN=y
 CONFIG_ELF=y
 CONFIG_EXAMPLES_HELLO=m
+CONFIG_EXAMPLES_HELLOXX=m
 CONFIG_FS_HOSTFS=y
 CONFIG_FS_PROCFS=y
+CONFIG_HAVE_CXX=y
 CONFIG_IDLETHREAD_STACKSIZE=3072
 CONFIG_INIT_FILEPATH="/system/bin/init"
 CONFIG_INIT_MOUNT=y

Program output when helloxx is ran from nsh:

...
[    3.347000] elf_symvalue: SHN_UNDEF: Exported symbol "_Z8err_funciPKcz" not found
[    3.348000] elf_relocateadd: Section 2 reloc 77: Failed to get value of symbol[270]: -2
[    3.350000] elf_loadbinary: Failed to bind symbols program binary: -2
[    3.351000] exec_spawn: ERROR: Failed to load program 'helloxx': -2

Using objdump riscv64-unknown-elf-objdump apps/bin/helloxx -x | c++filt | less

SYMBOL TABLE:
...
...
00000000         *UND*  00000000 err_func(int, char const*, ...)

With the additions included in the PR, the symbol is no longed undefined with the application building and launching successfully.