Fix armv7l compilation (#30253)

* src/task.c: Use `bx` instead of `br` instruction on armv7l

* Fix typo and incorrect initialization within `jl_getauxval` on armv7l.

src/processor_arm.cpp

@@ -522,8 +522,8 @@ static inline unsigned long jl_getauxval(unsigned long type)
 {
     // First, try resolving getauxval in libc
     auto libc = jl_dlopen(nullptr, JL_RTLD_LOCAL);
-    static (unsigned long (*)(unsigned long) getauxval_p;
-    if (jl_dlsym(libc, "getauxval", &getauxval_p, 0) {
+    static unsigned long (*getauxval_p)(unsigned long) = NULL;
+    if (getauxval_p == NULL && jl_dlsym(libc, "getauxval", (void **)&getauxval_p, 0)) {
         return getauxval_p(type);
     }
 

src/task.c

@@ -762,7 +762,9 @@ static void jl_start_fiber(jl_ucontext_t *lastt, jl_ucontext_t *t)
     asm(" mov sp, %0;\n"
         " mov lr, #0;\n" // Clear link register (lr) and frame pointer
         " mov fp, #0;\n" // (fp) to terminate unwinder.
-        " br %1;\n" // call `fn` with fake stack frame
+        " bx %1;\n" // call `fn` with fake stack frame.  While `bx` can change
+                    // the processor mode to thumb, this will never happen
+                    // because all our addresses are word-aligned.
         " udf #0" // abort
         : : "r" (stk), "r"(fn) : "memory" );
 #else