Add aarch64-unknown-linux-musl as a cross-buildable architecture

This includes one of the gnarliest build hacks I've ever had to
perpetrate, as documented in
`crates/engine_xetex/xetex/xetex-engine-interface.c`.

Cross.toml

@@ -1,3 +1,6 @@
+[target.aarch64-unknown-linux-musl]
+image = "tectonictypesetting/crossbuild:aarch64-unknown-linux-musl"
+
 [target.arm-unknown-linux-musleabihf]
 image = "tectonictypesetting/crossbuild:arm-unknown-linux-musleabihf"
 

crates/engine_xetex/xetex/xetex-engine-interface.c

@@ -68,3 +68,45 @@ tt_engine_xetex_main(
     ttbc_global_engine_exit();
     return rv;
 }
+
+
+/* "What is happening here?", you might ask. Good question!
+ *
+ * My first attempt (PKGW, 2023 Sep) to build a static version of Tectonic
+ * for the aarch64 platform using Tectonic's cross-compilation framework
+ * ran into the following error when trying to link the final executable:
+ *
+ *   /home/rust/sysroot-aarch64/usr/lib/libc.a(sigsetjmp.lo): in function `sigsetjmp':
+ *   /home/buildozer/aports/main/musl/src/v1.2.3/src/signal/aarch64/sigsetjmp.s:7:(.text+0x0):
+ *     relocation truncated to fit: R_AARCH64_CONDBR19 against symbol `setjmp'
+ *     defined in .text section in /home/rust/sysroot-aarch64/usr/lib/libc.a(setjmp.lo)
+ *
+ * So, musl libc's implementation of sigsetjmp() invokes setjmp() in some
+ * hand-written assembly. It appears that for whatever reason, when the linker
+ * is trying to build the Tectonic executable on aarch64, it ends up wanting to
+ * locate setjmp() and sigsetjmp() far away from each other in the final file,
+ * and the particular branch instruction used in sigsetjmp() can only specify a
+ * relatively small offset that cannot capture the location of setjmp(). The musl
+ * developers tentatively agree that this seems to be a bug in musl's
+ * implementation.
+ *
+ * I had the idea that maybe if I referenced both functions in my code, that
+ * would encourage the linker to place them closer to each other. And, guess
+ * what, it seems to work! Bananas!
+ *
+ * This hardly costs us anything so we don't bother to try to #ifdef it for the
+ * specific circumstances given above, but Windows doesn't provide sigsetjmp.
+ */
+#ifndef _WIN32
+void  __terrible_aarch64_musl_linker_hack_never_call_me(void);
+
+void
+__terrible_aarch64_musl_linker_hack_never_call_me(void)
+{
+    jmp_buf buf1;
+    sigjmp_buf buf2;
+
+    setjmp(buf1);
+    sigsetjmp(buf2, 0);
+}
+#endif

dist/azure-build-and-test.yml

@@ -165,6 +165,10 @@ parameters:
 - name: crossBuilds
   type: object
   default:
+  - name: aarch64_unknown_linux_musl
+    vars:
+      TARGET: aarch64-unknown-linux-musl
+
   - name: arm_unknown_linux_musleabihf
     vars:
       TARGET: arm-unknown-linux-musleabihf