configure: use __ARM_ARCH to determine arm version

[joaocgreis]

Before this change, configure used processor specific macro defines (like __ARM_ARCH_6M__) to detect the arm processor version. This changes configure to use __ARM_ARCH, that should be defined to the correct version.

This is useful because configure may not expect the flag for a specific processor. Currently, if trying to cross compile optimizing to the Raspberry Pi 1 processor with -mcpu=arm1176jzf-s the detected arm_version will be default (because it sets__ARM_ARCH_6ZK__), and the generated binaries useless in that processor.

@nodejs/platform-arm ARM processors currently on CI are ok with this change, but could there be some other processor or compiler that does not set __ARM_ARCH or sets it to something that we do not want? @mmalecki you authored the original function, do you know of any problem with this? (Thanks!)

cc @nodejs/build

For reference:

$ echo | arm-linux-gnueabihf-gcc -dM -E - | grep ARM_ARCH
#define __ARM_ARCH_ISA_ARM 1
#define __ARM_ARCH_6__ 1
#define __ARM_ARCH_ISA_THUMB 1
#define __ARM_ARCH 6
$ echo | arm-linux-gnueabihf-gcc -mcpu=arm1176jzf-s -dM -E - | grep ARM_ARCH
#define __ARM_ARCH_ISA_ARM 1
#define __ARM_ARCH_ISA_THUMB 1
#define __ARM_ARCH 6
#define __ARM_ARCH_6ZK__ 1
$ echo | arm-linux-gnueabihf-gcc -mcpu=cortex-a7 -dM -E - | grep ARM_ARCH                                                    
#define __ARM_ARCH_ISA_ARM 1
#define __ARM_ARCH_PROFILE 65
#define __ARM_ARCH_ISA_THUMB 2
#define __ARM_ARCH 7
#define __ARM_ARCH_7A__ 1
#define __ARM_ARCH_EXT_IDIV__ 1

[bnoordhuis]

You could simplify this to return cc_macros_cache.get('__ARM_ARCH') == '7'.

[bnoordhuis]

LGTM with a comment.

[mmalecki]

@joaocgreis Not aware of such, no.

[jasnell]

CI: https://ci.nodejs.org/job/node-test-pull-request/915/

[joaocgreis]

Updated, thanks for the reviews! Will land Tuesday if there are no further comments.
CI with the changes: https://ci.nodejs.org/job/node-test-pull-request/944/

[joaocgreis]

Note: armv7-wheezy detects arm_version as 6, but that already happened before. I assume it's because the default compiler targets armv6, as this is what happens in the Raspberry Pi 2.

[rvagg]

armv7-wheezy isn't Raspbian fwiw, it's proper Wheezy running on some ARMv7 machines.

[joaocgreis]

Landed in v4.x-staging in 88c17f8