[build] Work around bug in gcc >= 4.8

gcc 4.8 and 4.9 fail to compile pxe_call.c with the error "bp cannot
be used in asm here".  Other points in the codebase which use "ebp" in
the asm clobber list do not seem to be affected.

Unfortunately gcc provides no way to specify %ebp as an output
register, so we cannot use this as a workaround.  The only viable
solution is to explicitly push/pop %ebp within the asm itself.  This
is ugly for two reasons: firstly, it may be unnecessary; secondly, it
may cause gcc to generate invalid %esp-relative addresses if the asm
happens to use memory operands.  This specific block of asm uses no
memory operands and so will not generate invalid code.

Reported-by: Daniel P. Berrange <berrange@redhat.com>
Reported-by: Christian Hesse <list@eworm.de>
Originally-fixed-by: Christian Hesse <list@eworm.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>