gdb/ser-unix: fix musl build failure when setting custom baud rates

On musl-based systems, <asm/termbits.h> may expose BOTHER even though
struct termios does not define c_ispeed/c_ospeed.  This causes the
Linux-specific custom baud rate path to be compiled and fail to build.

Fix the problem at the macro level by requiring
HAVE_STRUCT_TERMIOS_C_OSPEED (obtained via AC_CHECK_MEMBERS) together
with BOTHER in the HAVE_CUSTOM_BAUDRATE_SUPPORT guard.  This prevents
the Linux-specific code from being compiled on musl while leaving
set_custom_baudrate_linux unchanged.

This is a pure build fix with no functional or behavioural change on
any existing platform.

Suggested-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Sunil Dora <sunilkumar.dora@windriver.com>
Approved-by: Kevin Buettner <kevinb@redhat.com>