acpi: Add EBDA address sanity checks and work around bogus GCC warning

.github/workflows/check.yml

@@ -20,3 +20,6 @@ jobs:
 
       - name: Build the bootloader (LLVM)
         run: ./bootstrap && ./configure TOOLCHAIN=llvm --enable-werror --enable-all && make all && make maintainer-clean
+
+      - name: Build the bootloader (GCC)
+        run: ./bootstrap && ./configure LIMINE_CC=gcc LIMINE_LD=ld LIMINE_OBJCOPY=objcopy LIMINE_OBJDUMP=objdump LIMINE_READELF=readelf --enable-werror --enable-all && make all && make maintainer-clean

common/lib/acpi.h

@@ -3,8 +3,20 @@
 
 #include <stdint.h>
 #include <stddef.h>
+#include <sys/cpu.h>
 
-#define EBDA ((size_t)(*((volatile uint16_t *)0x40e)) * 16)
+#define EBDA (ebda_get())
+
+static inline uintptr_t ebda_get(void) {
+    uintptr_t ebda = (uintptr_t)mminw(0x40e) << 4;
+
+    // Sanity checks
+    if (ebda < 0x80000 || ebda >= 0xa0000) {
+        ebda = 0x80000;
+    }
+
+    return ebda;
+}
 
 struct sdt {
     char     signature[4];