ggml: fix arm build with gcc

[angt]

So, we continue :)

GCC doesn't like -mcpu=native+ext (and not -march=native+ext either):

$ gcc -mcpu=native+dotprod -o dotprod dotprod.c
cc1: error: unknown value ‘native+dotprod’ for ‘-mcpu’
cc1: note: valid arguments are: cortex-a34 cortex-a35 cortex-a53 cortex-a57 ...

The new detection mechanism doesn't work on my graviton4 (but luckily -mcpu=native works):

-- ARM detected
-- Performing Test COMPILER_SUPPORTS_FP16_FORMAT_I3E
-- Performing Test COMPILER_SUPPORTS_FP16_FORMAT_I3E - Failed
-- Performing Test GGML_COMPILER_SUPPORT_DOTPROD
-- Performing Test GGML_COMPILER_SUPPORT_DOTPROD - Failed
-- Performing Test GGML_COMPILER_SUPPORT_I8MM
-- Performing Test GGML_COMPILER_SUPPORT_I8MM - Failed
-- ARM feature DOTPROD enabled
-- ARM feature SVE enabled
-- ARM feature MATMUL_INT8 enabled
-- ARM feature FMA enabled
-- ARM feature FP16_VECTOR_ARITHMETIC enabled
-- Adding CPU backend variant ggml-cpu: -mcpu=native 

This PR propose to use -mcpu=native to detect the CPU like this:

gcc -mcpu=native -E -v - </dev/null 2>&1 | grep -oE "\-mcpu=[^ ']+" -m 1
-mcpu=neoverse-v2+crc+sve2-aes+sve2-sha3

With this PR we get:

GCC:

-- ARM detected
-- Performing Test COMPILER_SUPPORTS_FP16_FORMAT_I3E
-- Performing Test COMPILER_SUPPORTS_FP16_FORMAT_I3E - Failed
-- Performing Test GGML_COMPILER_SUPPORT_DOTPROD
-- Performing Test GGML_COMPILER_SUPPORT_DOTPROD - Success
-- Performing Test GGML_COMPILER_SUPPORT_I8MM
-- Performing Test GGML_COMPILER_SUPPORT_I8MM - Success
-- ARM feature DOTPROD enabled
-- ARM feature SVE enabled
-- ARM feature MATMUL_INT8 enabled
-- ARM feature FMA enabled
-- ARM feature FP16_VECTOR_ARITHMETIC enabled
-- Adding CPU backend variant ggml-cpu: -mcpu=neoverse-v2+crc+sve2-aes+sve2-sha3+dotprod+i8mm 

clang:

-- ARM detected
-- Performing Test COMPILER_SUPPORTS_FP16_FORMAT_I3E
-- Performing Test COMPILER_SUPPORTS_FP16_FORMAT_I3E - Failed
-- ARM -mcpu not found, -mcpu=native will be used
-- Performing Test GGML_COMPILER_SUPPORT_DOTPROD
-- Performing Test GGML_COMPILER_SUPPORT_DOTPROD - Success
-- Performing Test GGML_COMPILER_SUPPORT_I8MM
-- Performing Test GGML_COMPILER_SUPPORT_I8MM - Success
-- ARM feature DOTPROD enabled
-- ARM feature SVE enabled
-- ARM feature MATMUL_INT8 enabled
-- ARM feature FMA enabled
-- ARM feature FP16_VECTOR_ARITHMETIC enabled
-- Adding CPU backend variant ggml-cpu: -mcpu=native+dotprod+i8mm

Where -mcpu=native+ext works...

[angt]

I was tricked by my mac ^^

$ gcc --version
Apple clang version 16.0.0 (clang-1600.0.26.6)
Target: arm64-apple-darwin24.1.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

[slaren]

I tried testing on M3 Max with gcc from brew, but it does not seem to work very well, it fails during the assembler with "error: instruction requires: i8mm". So I guess no gcc on Apple.