[mpdecimal] Enable all platforms

Some platforms are currently disabled in the mpdecimal package.

The segfaults on these platforms in the libmpdec++ tests are a consequence of the relatively small default thread stack size of musl (128K).

The tests use std::thread by default and assume at least 300K. std::thread cannot adjust the stack size.

For platforms with a low default stack size (e.g. AIX has only 96K) there is an option to use pthreads instead of std::thread in the tests.

To be clear, both libmpdec++ and musl are fine, the end user application needs to take steps to avoid stack overflows.

With this diff the tests pass on Alpine i386:

--- a-mpdecimal-2.5.1/tests++/runshort.sh
+++ b-mpdecimal-2.5.1/tests++/runshort.sh
@@ -12,9 +12,11 @@
         export MallocLogFile=/dev/null
         export MallocDebugReport=crash
         ;;
-    AIX)
+    AIX|Linux)
         # The thread stack size on AIX (96K) makes std::thread unusable
         # for runtest.cc, which needs around 300K.
+        #
+        # The same applies to Alpine Linux (musl).
         THREAD="--pthread"
         ;;
     *)
--- a-mpdecimal-2.5.1/tests++/runshort_alloc.sh
+++ b-mpdecimal-2.5.1/tests++/runshort_alloc.sh
@@ -12,9 +12,11 @@
         export MallocLogFile=/dev/null
         export MallocDebugReport=crash
         ;;
-    AIX)
+    AIX|Linux)
         # The thread stack size on AIX (96K) makes std::thread unusable
         # for runtest.cc, which needs around 300K.
+        #
+        # The same applies to Alpine Linux (musl).
         THREAD="--pthread"
         ;;
     *)