The gthr weak reference based single thread detection is unsafe with
static linking and in case of dynamic linking it's ineffective on musl
since pthread symbols are defined in libc.so.

(Ideally this should be fixed for all targets, since glibc plans to move
libpthread.so into libc.so too and users want to static link to pthread
without --whole-archive: PR87189.)

For now we have to explicitly opt out from the broken behaviour in the
config machinery of each target lib and libgcc was previously missed.

libgcc/ChangeLog:

2019-11-18  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* config.host: Add t-gthr-noweak on *-*-musl*.
	* config/t-gthr-noweak: New file.

From-SVN: r278399