Fix "cannot find the function: strcpy()" error on arm32 on Travis CI. (#2686)

This issue happened when `libc.so` and `libm.so` path were not found
and `ldd ruby` command also failed to print the shared dependencies
in `test/fiddle/helper.rb`.

See https://travis-ci.org/ruby/ruby/jobs/611483288#L3018
/home/travis/build/ruby/ruby/build/.ext/common/fiddle/import.rb:299:in `import_function': cannot find the function: strcpy() (Fiddle::DLError)

* Set libc6:armhf as a installing dependency explicitly.
* Remove arm32 from allow_failures.

.travis.yml

@@ -304,6 +304,7 @@ env:
         tool/travis_retry.sh sudo -E apt-get $travis_apt_get_options install \
           ccache \
           crossbuild-essential-armhf \
+          libc6:armhf \
           libstdc++-5-dev:armhf \
           libffi-dev:armhf \
           libffi6:armhf \
@@ -432,7 +433,6 @@ matrix:
     - <<: *CALL_THREADED_CODE
     - <<: *NO_THREADED_CODE
   allow_failures:
-    - name: arm32-linux
     - name: -fsanitize=address
     - name: -fsanitize=memory
     - name: -fsanitize=undefined

test/fiddle/helper.rb

@@ -22,7 +22,16 @@ when /linux/
   case RbConfig::SIZEOF['void*']
   when 4
     # 32-bit ruby
-    libdir = '/lib32' if File.directory? '/lib32'
+    case RUBY_PLATFORM
+    when /armv\w+-linux/
+      # In the ARM 32-bit libc package such as libc6:armhf libc6:armel,
+      # libc.so and libm.so are installed to /lib/arm-linux-gnu*.
+      # It's not installed to /lib32.
+      dirs = Dir.glob('/lib/arm-linux-gnu*')
+      libdir = dirs[0] if dirs && File.directory?(dirs[0])
+    else
+      libdir = '/lib32' if File.directory? '/lib32'
+    end
   when 8
     # 64-bit ruby
     libdir = '/lib64' if File.directory? '/lib64'
@@ -104,6 +113,9 @@ libm_so = nil if !libm_so || (libm_so[0] == ?/ && !File.file?(libm_so))
 
 if !libc_so || !libm_so
   ruby = EnvUtil.rubybin
+  # When the ruby binary is 32-bit and the host is 64-bit,
+  # `ldd ruby` outputs "not a dynamic executable" message.
+  # libc_so and libm_so are not set.
   ldd = `ldd #{ruby}`
   #puts ldd
   libc_so = $& if !libc_so && %r{/\S*/libc\.so\S*} =~ ldd