powerpc/boot: Delete unneeded .globl _zimage_start

.globl sets the symbol binding to STB_GLOBAL while .weak sets the
binding to STB_WEAK. GNU as let .weak override .globl since
binutils-gdb 5ca547dc2399a0a5d9f20626d4bf5547c3ccfddd (1996). Clang
integrated assembler let the last win but it may error in the future.

Since it is a convention that only one binding directive is used, just
delete .globl.

Fixes: ee9d21b3 ("powerpc/boot: Ensure _zimage_start is a weak symbol")
Signed-off-by: Fangrui Song <maskray@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200325164257.170229-1-maskray@google.com

arch/powerpc/boot/crt0.S

@@ -44,9 +44,6 @@ p_end:		.long	_end
 p_pstack:	.long	_platform_stack_top
 #endif
 
-	.globl	_zimage_start
-	/* Clang appears to require the .weak directive to be after the symbol
-	 * is defined. See https://bugs.llvm.org/show_bug.cgi?id=38921  */
 	.weak	_zimage_start
 _zimage_start:
 	.globl	_zimage_start_lib