Merge bitcoin-core/secp256k1#1304: build: Rename arm to arm32 and check if it's really supported

c6bb29b303 build: Rename `64bit` to `x86_64` (Hennadii Stepanov)
03246457a8 autotools: Add `SECP_ARM32_ASM_CHECK` macro (Hennadii Stepanov)
ed4ba238e2 cmake: Add `check_arm32_assembly` function (Hennadii Stepanov)
e5cf4bf3ff build: Rename `arm` to `arm32` (Hennadii Stepanov)

Pull request description:

  Closes https://github.com/bitcoin-core/secp256k1/issues/1034.

  Solves one item in https://github.com/bitcoin-core/secp256k1/issues/1235.

ACKs for top commit:
  real-or-random:
    ACK c6bb29b303 tested on x86_64 but not on ARM

Tree-SHA512: c3615a18cfa30bb2cc53be18c09ccab08fc800b84444d8c6b333347b4db039a3981da61e7da5086dd9f4472838d7c031d554be9ddc7c435ba906852bba593982

.cirrus.yml

@@ -176,7 +176,7 @@ task:
     CTIMETESTS: no
   matrix:
     - env: {}
-    - env: {EXPERIMENTAL: yes, ASM: arm}
+    - env: {EXPERIMENTAL: yes, ASM: arm32}
   << : *MERGE_BASE
   test_script:
     - ./ci/cirrus.sh