Merge #1242: Set ARM ASM symbol visibility to hidden

fd2a408 Set ARM ASM symbol visibility to `hidden` (Hennadii Stepanov)

Pull request description:

  Solves one item in #1181.

  To test on arm-32bit hardware, run:
  ```
  $ ./autogen.sh && ./configure --enable-experimental --with-asm=arm && make
  ```

  On master branch (427bc3c):
  ```
  $ nm -D .libs/libsecp256k1.so | grep secp256k1_fe
  0000e2bc T secp256k1_fe_mul_inner
  0000e8dc T secp256k1_fe_sqr_inner
  ```

  With this PR:
  ```
  $ nm -D .libs/libsecp256k1.so | grep secp256k1_fe | wc -l
  0
  ```

  For reference, see https://sourceware.org/binutils/docs/as/Hidden.html.

ACKs for top commit:
  theuni:
    ACK fd2a408.
  sipa:
    ACK fd2a408

Tree-SHA512: abf8ad332631672c036844f69c5599917c49e12c4402bf9066f93a692d3007b1914bd3eea8f83f0141c1b09d5c88ebc5e6c8bfbb5444b7b3471749f7b901ca59

src/asm/field_10x26_arm.s

@@ -29,6 +29,7 @@ Note:
 	.align	2
 	.global secp256k1_fe_mul_inner
 	.type	secp256k1_fe_mul_inner, %function
+	.hidden secp256k1_fe_mul_inner
 	@ Arguments:
 	@  r0  r      Restrict: can overlap with a, not with b
 	@  r1  a
@@ -516,6 +517,7 @@ secp256k1_fe_mul_inner:
 	.align	2
 	.global secp256k1_fe_sqr_inner
 	.type	secp256k1_fe_sqr_inner, %function
+	.hidden secp256k1_fe_sqr_inner
 	@ Arguments:
 	@  r0  r	 Can overlap with a
 	@  r1  a