README: bump the gcc baseline to 8.4

We already use __VA_OPT__ in multiple places, which was introduced in
gcc 8 [0], so let's bump the baseline to reflect that. I chose gcc 8.4,
as that was the lowest 8.x version I could easily get my hands on when I
verified this (on Ubuntu Focal with the gcc-8 package).

Closes: #31191

[0] https://gcc.gnu.org/gcc-8/changes.html

README

@@ -217,7 +217,7 @@ REQUIREMENTS:
         pyelftools (optional, required for systemd-boot)
         meson >= 0.60.0
         ninja
-        gcc >= 4.7
+        gcc >= 8.4
         awk, sed, grep, and similar tools
         clang >= 10.0, llvm >= 10.0 (optional, required to build BPF programs
                 from source code in C)