[gdb/build] Fix build breaker with --enable-shared

[gdb/build] Fix build breaker with --enable-shared

When building gdb with --enable-shared, I run into:
...
ld: build/zlib/libz.a(libz_a-inffast.o): relocation R_X86_64_32S against \
  `.rodata' can not be used when making a shared object; recompile with -fPIC
ld: build/zlib/libz.a(libz_a-inflate.o): warning: relocation against \
  `inflateResetKeep' in read-only section `.text'
collect2: error: ld returned 1 exit status
make[3]: *** [libbfd.la] Error 1
...

This is a regression since commit a08bdb159bb ("[gdb/build] Fix gdbserver
build with -fsanitize=thread").

The problem is that a single case statement in configure is shared to handle
special requirements for both the host libiberty and host zlib, which has the
effect that only one is handled.

Fix this by handling libiberty and zlib each in its own case statement.

Build on x86_64-linux, with and without --enable-shared.

ChangeLog:

2022-06-27  Tom de Vries  <tdevries@suse.de>

	* configure.ac: Set extra_host_libiberty_configure_flags and
	extra_host_zlib_configure_flags in separate case statements.
	* configure: Regenerate.