crypto: arm/curve25519 - Move '.fpu' after '.arch'

Debian's clang carries a patch that makes the default FPU mode
'vfp3-d16' instead of 'neon' for 'armv7-a' to avoid generating NEON
instructions on hardware that does not support them:

https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/-/raw/5a61ca6f21b4ad8c6ac4970e5ea5a7b5b4486d22/debian/patches/clang-arm-default-vfp3-on-armv7a.patch
https://bugs.debian.org/841474
https://bugs.debian.org/842142
https://bugs.debian.org/914268

This results in the following build error when clang's integrated
assembler is used because the '.arch' directive overrides the '.fpu'
directive:

arch/arm/crypto/curve25519-core.S:25:2: error: instruction requires: NEON
 vmov.i32 q0, #1
 ^
arch/arm/crypto/curve25519-core.S:26:2: error: instruction requires: NEON
 vshr.u64 q1, q0, #7
 ^
arch/arm/crypto/curve25519-core.S:27:2: error: instruction requires: NEON
 vshr.u64 q0, q0, #8
 ^
arch/arm/crypto/curve25519-core.S:28:2: error: instruction requires: NEON
 vmov.i32 d4, #19
 ^

Shuffle the order of the '.arch' and '.fpu' directives so that the code
builds regardless of the default FPU mode. This has been tested against
both clang with and without Debian's patch and GCC.

Cc: [email protected]
Fixes: d8f1308 ("crypto: arm/curve25519 - wire up NEON implementation")
Link: https://github.com/ClangBuiltLinux/continuous-integration2/issues/118
Reported-by: Arnd Bergmann <[email protected]>
Suggested-by: Arnd Bergmann <[email protected]>
Suggested-by: Jessica Clarke <[email protected]>
Signed-off-by: Nathan Chancellor <[email protected]>
Acked-by: Jason A. Donenfeld <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Tested-by: Nick Desaulniers <[email protected]>
Signed-off-by: Herbert Xu <[email protected]>

arch/arm/crypto/curve25519-core.S

@@ -10,8 +10,8 @@
 #include <linux/linkage.h>
 
 .text
-.fpu neon
 .arch armv7-a
+.fpu neon
 .align 4
 
 ENTRY(curve25519_neon)