x86/boot: Don't overwrite cr4 when enabling PAE

Some EFI firmware makes use of the FPU during boottime services and
clearing X86_CR4_OSFXSR by overwriting %cr4 causes the firmware to
crash.

Add the PAE bit explicitly instead of trashing the existing contents,
leaving the rest of the bits as the firmware set them.

Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: NMatt Fleming <matt.fleming@intel.com>

arch/x86/boot/compressed/head_64.S

@@ -113,7 +113,8 @@ ENTRY(startup_32)
 	lgdt	gdt(%ebp)
 
 	/* Enable PAE mode */
-	movl	$(X86_CR4_PAE), %eax
+	movl	%cr4, %eax
+	orl	$X86_CR4_PAE, %eax
 	movl	%eax, %cr4
 
  /*