Fix musl build for bluetooth

Turning on code coverage directly with cflags and ldflags is not
supported, it generally takes more work from the build system to
provide the right runtime.  It happens to work for glibc, where
clang can guess the right runtime itself, but when building against
musl clang picks the wrong runtime causing link errors:

ld.lld: error: undefined symbol: __strdup
>>> referenced by InstrProfilingFile.c
>>>               InstrProfilingFile.c.o:(parseAndSetFilename) in archive /usr/local/google/home/ccross/ssd/aosp/prebuilts/clang/host/linux-x86/clang-r450784e/lib64/clang/14.0.7/lib/linux/libclang_rt.profile-x86_64.a

Move the flags to only be used with glibc.

Also include sys/cdefs.h for __BEGIN_DECLS and __END_DECLS.

Bug: 190084016
Test: m USE_HOST_MUSL=true
Change-Id: I1eef4aee5c7b691c0638cc1b00ce4b4661397974