OvmfPkg: Don't add -mno-mmx -mno-sse option for XCODE5 tool chain

OvmfPkg: Don't add -mno-mmx -mno-sse option for XCODE5 tool chain

Ovmf appended option -mno-mmx -mno-sse, but these two options were enabled
in Openssl. The compiler option becomes -mmmx ?msse -mno-mmx -mno-sse. It
trig mac clang compiler hang when compile one source file in openssl.
This issue is found when SECURE_BOOT_ENABLE is TRUE. This may be the compiler
issue. To work around it, don't add these two options for XCODE5 tool chain.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>