clang: error: unsupported option '-ffixed-x18' for target 'arm-linux-gnueabi'

[Jack Andrews]

thanks for uboot!

i'm trying to build uboot for raspi 4.
these steps produced warnings and a compile error
```
$ git clone https://source.denx.de/u-boot/u-boot.git
$ cd u-boot/
$ make CC=clang rpi_4_defconfig
  HOSTCC  scripts/basic/fixdep
  HOSTCC  scripts/kconfig/conf.o
  YACC    scripts/kconfig/zconf.tab.c
  LEX     scripts/kconfig/zconf.lex.c
  HOSTCC  scripts/kconfig/zconf.tab.o
  HOSTLD  scripts/kconfig/conf
generated_defconfig:19:warning: unexpected data:  # CONFIG_DISPLAY_CPUINFO
is not set
generated_defconfig:20:warning: unexpected data:  #
CONFIG_DISPLAY_BOARDINFO is not set
generated_defconfig:46:warning: unexpected data:  # CONFIG_PINCTRL_GENERIC
is not set
generated_defconfig:49:warning: unexpected data:  #
CONFIG_REQUIRE_SERIAL_CONSOLE is not set
generated_defconfig:64:warning: unexpected data:  # CONFIG_VIDEO_BPP8 is
not set
generated_defconfig:65:warning: unexpected data:  # CONFIG_VIDEO_BPP16 is
not set
generated_defconfig:70:warning: unexpected data:  # CONFIG_HEXDUMP is not
set
#
# configuration written to .config
#
$ make HOSTCC=clang CROSS_COMPILE=arm-linux-gnueabi- CC=clang
  HOSTCC  scripts/kconfig/conf.o
  HOSTCC  scripts/kconfig/zconf.tab.o
  HOSTLD  scripts/kconfig/conf
scripts/kconfig/conf  --syncconfig Kconfig
  UPD     include/config.h
  CFG     u-boot.cfg
clang: error: unsupported option '-ffixed-x18' for target
'arm-linux-gnueabi'
make[1]: *** [scripts/Makefile.autoconf:82: u-boot.cfg] Error 1
make: *** No rule to make target 'include/config/auto.conf', needed by
'include/config/uboot.release'.  Stop.
```
```
$ clang -v
Ubuntu clang version 18.1.3 (1ubuntu1)
...
```